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I'm a new member and this is my first post.
I am based in the UK and I inadvertently bought a 1978 C3 project. Much as I'd like to be "hands on" with the restoration I have neither the time or the ability to see the job through and the car is currently with a friendly local garage who have a reputation for doing good work.
They have got some good work done but they have run into a problem with the diff - it's cracked! I will attach some pictures.
I would be grateful for some advice on the best way forward to make a good repair. Have read some of the other threads on here it seems like I need to speak to Gary/GTV1999, but, as a new member, I am unable to write him a DM.
I am in America next week, and, depending on how heavy they are, I could take some parts home in my suitcase.
Thanks
That looks like it could be a problem. Lol. They're readily available stock and aftermarket. I bought that entire unit from Yukon gear for 700 bucks. Great deal, stronger than stock. Fit like a glove.
Cracked posi case, this one let go before caught. It happens.
Yukon cases are not as good as Eaton, in my opinion. Both imported, both use lousy clutches.
USA made Eatons are thicker at the window and better than both new Eaton and Yukon, but also obsoleted now. The Eaton cases are machined better-in China- then the original USA made cases installed in the cars. It kills me to say that but it's true.
The assembly of a new out of the box posi has to be checked. I never used them out of the box, they always come apart and I rebuild them before I will use them. Many times, the assembly is sloppy, but with fiber clutches and huge springs, you will never know. At every turn you are smudging the fiber off into the oil. It will black in no time. They use one less clutch plate and disc per side.
I used 3 new ones over the past month. Prices keep going up and backorders are common.
78 diff's also had bad axles and ring gear bolts. You might save money finding a good used core with a good posi case.
Gary is the resident expert on these differentials and will build you the best unit possible but given you are located in the UK, it becomes very problematic for you logistically and financially to get the car back on the road.
if I were you, I’d first ask you shop to make a full assessment of what you have. Buying another used C3 differential is a Forrest Gump exercise. Often, it is better to deal with the devil you know than one you don’t. Make sure the main housing is undamaged. Carefully check the ring and pinion for damage. Measure the dimension of the stub axles from the top of the retention clip groove to the axle tip - should be 0.185” +/-. Many, many axles have badly worn due to improper heat treating so don’t be shocked to find a lot less. If you are lucky and the axles tips don’t exhibit wear, measure the hardness of the axle.
While you can try to find a good used posi unit, you are probably better off with a new “loaded unit”. Yes, it will have fiber clutches and yes, it will cause the oil to turn black but it will function and you will be able to enjoy the car lot sooner.
Have your shop view the Tom’s Eaton Posi Tuneup YouTube videos to see how to setup the posi with zero slack in the spider gears. Then you can ignore reinstalling the spring pack. Or, you can have them bolt it in as-is and not lose any sleep.
Ensure the shop uses Timken bearings. I buy mine from Denny’s Driveshaft in New York Stat
A completely bare posi case is only 11.1 lbs. So It ?could? maybe go in a large suitcase. Or separate box. Fully loaded unit goes 18.5 lbs.
Maybe you could find a good crack free core at one of the Corvette Suppliers while you are here, and drag it home. Or a new one.
Zip, Bairs, Corvette Paramedics, Corvette America, Paragon, there are quite a few, just depends on what state you will be in.
Where will you be?
Maybe one of us could suggest a local source.
A completely bare posi case is only 11.1 lbs. So It ?could? maybe go in a large suitcase. Or separate box. Fully loaded unit goes 18.5 lbs.
Maybe you could find a good crack free core at one of the Corvette Suppliers while you are here, and drag it home. Or a new one.
Zip, Bairs, Corvette Paramedics, Corvette America, Paragon, there are quite a few, just depends on what state you will be in.
Where will you be?
Maybe one of us could suggest a local source.
Hi Leigh
Thank you for that - I am going to be in Las Vegas. 18.5lbs is well within my weight allowance, so it is definitely a possibility, if it will physically fit. I guess I can try the old one for size....
I would not think the posi units are “on the shelf” at any local Vegas business. You likely will need to order the unit from one of the suppliers and expedite ship to your hotel.
Summit has a store in Sparks NV about 8 hours away from Vegas? You might be able to buy one and have it shipped to your Hotel. It is loaded and $700 or so. 10 x 10 x10 box probably 20 lb. Trying to find a good used posi is a crap shoot. I just posted I thought I had a good one and an hour into it I found a small crack.
Summit has a store in Sparks NV about 8 hours away from Vegas? You might be able to buy one and have it shipped to your Hotel. It is loaded and $700 or so. 10 x 10 x10 box probably 20 lb. Trying to find a good used posi is a crap shoot. I just posted I thought I had a good one and an hour into it I found a small crack.
Not for me.
Think about all the work to remove the diff, then go through it and build it better than the common junk sold today. I get to a cracked posi and want to risk it to save some upfront money? If I was a liar and conman as some are, I wouldn't even tell the customer there is a crack, wouldn't polish or tune it, and use it. When it broke, I would tell them to send it back for another reRIpbuild. Sound crazy- it's been happening for many years. Perfect example of a current "race vendor", selling a rebuilt diff to a guy who never installed it. He brought it to me years ago to check and look what was under the cover. That same place is still in business and some here just bought diff's from them.
“No man has a right to expect to succeed in life unless he understands his business, and nobody can understand his business thoroughly unless he learns it by personal application and experience.” – P.T. Barnum
Not for me.
Think about all the work to remove the diff, then go through it and build it better than the common junk sold today. I get to a cracked posi and want to risk it to save some upfront money? If I was a liar and conman as some are, I wouldn't even tell the customer there is a crack, wouldn't polish or tune it, and use it. When it broke, I would tell them to send it back for another reRIpbuild. Sound crazy- it's been happening for many years. Perfect example of a current "race vendor", selling a rebuilt diff to a guy who never installed it. He brought it to me years ago to check and look what was under the cover. That same place is still in business and some here just bought diff's from them.
“No man has a right to expect to succeed in life unless he understands his business, and nobody can understand his business thoroughly unless he learns it by personal application and experience.” – P.T. Barnum
Yeah welcome to my world. You would really say WOW if you saw 1/2 of what I see come in that was "rebuilt"
The real kicker with the cracked posi above- the car owner called the business owner to ask how a 2" cracked posi was used when rebuilding the diff. How the diff was never installed and only by chance of opening it was it found. The answer- "you have a 45-50 year old posi, what do you expect"
So if I were you I would order a new Eaton posi now and have it shipped to your Hotel. I have done such a thing before. They can hold it for you.
Since you have a week lead time and a week in the US it might work just fine.
Your next step is to have the shop put the side yokes in the posi with the center pin and see how much slop there is. Gary likes .006" inch. Many wear and have .250 or more.
Or measure the yokes from the top of the c-clip groove to the top of the shaft. A decent not too worn side yoke should measure around .185".
If yours are badly worn, and 78s tend to do that, you may need new side yokes as well. And good ones are harder to find than posi units. But at least they only weigh 5.8 lbs each.
These are about .185" Sometimes the beveled end is worn all the way off, or even also the entire c-clip groove.
Thank you all for your contributions - I will discuss them all with the garage and report back.
Milt
I still have my original one. It needs rebuild but is in great shape. I only took it out because a thrust washer was cracked. Rather than rebuild I bought the Yukon one. It's been great, I don't drive at the race track so it's plenty more than I'll ever need. Unfortunately I'm in bc canada so can't get this chunk to you. But it's available if you know a way.
The real kicker with the cracked posi above- the car owner called the business owner to ask how a 2" cracked posi was used when rebuilding the diff. How the diff was never installed and only by chance of opening it was it found. The answer- "you have a 45-50 year old posi, what do you expect"
WOW! That is one of the worst cases of "customer service" I have ever heard of! I used to be responsible for a 24 person customer service department that handled 800 calls a day. Someone would get fired for a statement like that.
Differential problems
